Hot melt adhesives are adhesives that are able to adhere various products efficiently since they solidify in a short time, and are highly safe to human body since the adhesives do not require solvents. Therefore, hot melt adhesives are used in a variety of fields.
In regard to hot melt adhesives, it is known that various thermoplastic resins can be used as base polymers, but one of representative examples of the base polymers may be an aromatic vinyl-conjugated diene-aromatic vinyl triblock copolymer such as a styrene-isoprene-styrene triblock copolymer or a styrene-butadiene-styrene triblock copolymer. A hot melt adhesive using this aromatic vinyl-conjugated diene-aromatic vinyl triblock copolymer as a base polymer has excellent adaptability particularly to high-speed continuous production or excellent initial tack strength, and is therefore widely used. In this regard, various investigations have been conducted in order to improve various characteristics.
For example, Patent Literature 1 describes that when a tackifying agent, a diluent and a stabilizer are incorporated at particular proportions to an aromatic vinyl-conjugated diene-aromatic vinyl copolymer having aromatic vinyl copolymer blocks with mutually different molecular weights, a hot melt adhesive which has excellent low temperature adhesion level or the like and is suitable for the production of disposable products such as paper diapers may be obtained. Furthermore, Patent Literature 2 describes that when a hot melt adhesive composition is formed by incorporating a compatible polymer, a tackifying resin, a plasticizer and the like to a radial styrene-isoprene-styrene block copolymer, a hot melt adhesive composition which can be applied at a relatively low temperature and is suitable for label adhesion to bottles is obtained. Furthermore, Patent Literature 3 describes that when a pressure-sensitive adhesive is formed by incorporating a tackifying agent, a styrene-isoprene block copolymer, a plasticizer and the like to a styrene-isoprene-styrene block copolymer, a pressure-sensitive adhesive which can be easily cut by a die and is suitable for labeling is obtained. In addition, Patent Literature 4 describes that when an adhesive composition for labeling is formed by incorporating a particular polymer, a tackifying agent, a softening agent and the like to an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer having a particular structure, an adhesive composition for labeling which can be easily applied at low temperatures and has satisfactory die-cutting performance is obtained.
However, in regard to the adhesive compositions formed by incorporating a softening agent (diluent/plasticizer) and the like to a block copolymer as described in Patent Literatures 1 to 4, incorporation of a softening agent contributes to lowering of the coatable temperature or an increase in tackiness; however, when the amount of incorporation of the softening agent is increased, there is a problem that the softening agent bleeds out and contaminates the object to be adhered. Furthermore, in regard to the improved adhesive compositions described in Patent Literatures 1 to 4, in a case in which adhesion has been conducted under high temperature conditions, or in a case in which an adherend has been stored under high temperature conditions, there are occasions in which problems such as detachment of the object to be adhered or flowing out of the adhesive composition, may occur. Furthermore, there are also occasions in which the improved adhesive compositions described in Patent Literatures 1 to 4 deteriorate designability of an adherend due to the lack of transparency. Under such circumstances as described above, in hot melt adhesives using an aromatic vinyl-conjugated diene-aromatic vinyl triblock copolymer as a base polymer, there is a strong demand for improvements in the performance in which an incorporated softening agent does not easily bleed (softening agent holding properties), heat resistance and transparency.